ASCA fast mode data has no positional information, but the RAWY column
contains corrections to the time column. The fast task applies these
timing corrections to the time column of the file. The output is
written to the workspace, and can be used in further calls to
extract,...

To use fast, you need to have made an unbinned image in one of the
other detectors, or in the detector you are analysing if there is some
bright mode data available as well as the fast mode data. Fast will
ask you for the X-position of the source, and for the instrument from
which you got this information.